Conventional optical communication networks operate by sending light pulses of a predetermined period, for example such that a pulse represents a ‘1’ and no pulse represents a ‘0’. This technique enables signals to be sent at data rates of up to 10 Gb/s and wavelength division multiplexing (WDM) techniques can be used to send multiple signals over a single fiber. Dense WDM (DWDM) enables up to 160 wavelengths to be used such that a single fiber can potentially carry 1.6 Tb/s of data. In order to enable compatibility between network components from different vendors, the ITU has specified a grid of wavelengths that are used in DWDM systems (see ITU-T G.694.1). One of the transmission phenomena present in optical fibers is chromatic dispersion, which causes the transmitted pulse to spread out, such that it becomes difficult to recover the transmitted signal at the receiver. The effects of dispersion can be mitigated by installing dispersion compensating modules (DCMs) into the network, but this adds to the cost and the complexity of the network.
Coherent optical transmission systems are thought to provide the best option for transmitting data at a rate in excess of 40 Gb/s. Coherent optical transmission systems are similar to the transmission systems used in wireless systems. Rather than turning an optical transmitter on and off to generate a pulse, an optical signal is modulated, for example in terms of phase or frequency, with a data signal. When the optical signal is received it is then recovered using a local oscillator and the transmitted data can be obtained by demodulating the optical signal. Dispersion is less of a problem and can be compensated for electronically during the demodulation of the optical signal, so it will be seen that coherent optical transmission systems do not require DCMs to be installed in the network, and in fact they work better without.
As data transmission rates increase further, for example beyond 100 Gb/s, then the optical signals required to transmit such data rates may not fit into the grid of wavelengths that are defined in the DWDM specifications. It is preferred, for reasons of flexibility and spectral efficiency, that for such high data rates, network operators are able to determine which regions of the optical transmission window are used to transmit specific signals. For example, one signal may extend across the space in the window that is reserved for multiple DWDM wavelengths.